Biodegradable polymers having a sustained-release property are useful as matrices for microcapsules, etc. to be employed for encapsulating physiologically active substances. As such biodegradable polymers, there are known, for instance, polylactic acid and a copolymer of lactic acid and glycolic acid (e.g. JP-A-11/269094).
These biodegradable polymers are used just as produced by conventional synthetic procedures. However, it has been found that such polymers produced by ring-opening polymerization are small in the terminal carboxyl group content and have poor utilization as sustained-release matrices. Because of this reason, attempt has been made to subject biodegradable polymers of high molecular weight to hydrolysis for making their weight-average molecular weights suitable and then use as a matrix for sustained-release preparations. The polymers obtained by hydrolysis and subsequent water washing are, however, apt to cause initial burst and therefore not suitable for sustained-release matrices, even when said polymers have proper weight-average molecular weights and terminal carboxyl group contents. Further improvement is thus demanded.